Co-localization of c-Fos and neurotransmitter immunoreactivities in the cat brain stem after carotid sinus nerve stimulation

Activation of nitric oxide-producing neurons in the brain stem during cardiac sympathoexcitatory reflexes in the cat

Our previous studies have shown that selective inhibition of nitric oxide in the brain reduces pressor responses to activation of cardiac sympathetic afferents, thus suggesting that nitric oxide is involved in central regulation of cardiac-cardiovascular sympathoexcitatory reflexes. Central neural regions in which nitric oxide-producing neurons are activated during these reflexes have not been well characterized. In the present study, we located nitric oxide-producing neurons in the brain stem activated by the input from cardiac sympathetic afferents by detecting colocalization of c-Fos immunoreactivity with nitric oxide synthesizing neurons. Expression of c-Fos has been used as a marker of activated neurons. Nitric oxide-producing neurons were identified by histochemical labeling of nicotine adenine dinucleotide phosphate-diaphorase (NADPH-d). In anesthetized cats with bilateral barodenervation and cervical vagotomy, bradykinin (1-10 microg in 0.1 ml; n=6) was applied to the anterior surface of the left ventricle six times every 20 minutes. Repetitive application of bradykinin consistently increased blood pressure, while the vehicle for bradykinin (0.9% saline, n=5) produced no responses. A substantial fraction (6-27%) of NADPH-d positive neurons displayed Fos immunoreactivity in the nucleus of the solitary tract, caudal and rostral ventral lateral medulla, lateral tegmental field, locus coeruleus and parabrachial nucleus in the bradykinin-treated cats. However, either no or rare (1-4%) double-labeled cells were found in these regions in control animals. Thus, nitric oxide-producing neurons are activated in several regions in the brain stem during stimulation of cardiac sympathetic afferents by bradykinin. Our data suggest that nitric oxide functions as a neurotransmitter/modulator in these areas to regulate the cardiac sympathoexcitatory reflexes.

Properties of solitary tract neurones responding to peripheral arterial chemoreceptors

Despite the highly integrated pattern of response evoked by peripheral chemoreceptor stimulation, limited information exists regarding the neurones within the nucleus of the solitary tract that mediate this reflex. Using a working heart-brainstem preparation, we describe evoked synaptic response patterns, some intrinsic membrane properties, location, morphology and axonal projections of physiologically characterised 'chemoreceptive' neurones located in the solitary tract nucleus in the rat. From 172 whole cell recordings, 56 neurones were identified as chemoreceptive since they responded to aortic injections of low doses of sodium cyanide (2-5 microg). Chemoreceptive neurones had a mean resting membrane potential of -52+/-1 mV and input resistance was 297+/-15 M(Omega) (n=56). Synaptic responses evoked included excitatory synaptic potentials alone, excitatory-inhibitory post-synaptic potential complexes, inhibitory synaptic potentials alone and central respiratory modulated synaptic potentials. Synaptic response latency data were obtained by stimulating electrically the solitary tract: the mean excitatory synaptic latency was 5.2+/-0.4 ms (range 2.5-8.0 ms; n=17). Chemoreceptive neurones showed a heterogeneity in their intrinsic membrane properties: neurones displayed either steady state, augmenting or adapting firing responses to depolarising current injection and, in some neurones, either delayed excitation or rebound activity following hyperpolarising pulses. Eleven chemoreceptive neurones were labelled and provided the first morphological data of these cells. Labelled somata were detected dorsomedial or medial to the solitary tract spanning the obex. Neurones typically had three to eight primary dendrites which often entered the solitary tract as well as extending across the ipsilateral region of the nucleus of the solitary tract. Axons were mostly unmyelinated with boutons of the en passant variety and often ramified within the solitary tract nucleus as well as coursed towards the ipsilateral ventral medulla. In summary, this study provides new data on the neurophysiological, anatomical and morphological properties of nucleus of the solitary tract neurones responding to arterial chemoreceptors in the rat.

Effects of selective sinoaortic denervations on phenylephrine-induced activational responses in the nucleus of the solitary tract

Intravenous administration of phenylephrine provokes a pattern of cellular activation in the nucleus of the solitary tract that resembles the central distributions of primary baroreceptor afferents supplied by the carotid sinus and aortic depressor nerves. Transganglionic transport and denervation methods were used in an experimental setting to test the dependence of phenylephrine-induced Fos immunoreactivity on the integrity of buffer nerve afferents, and to identify the subregions of the nucleus of the solitary tract supplied by each. Cholera toxin B-horseradish peroxidase injections into either or both nerves revealed terminal labeling concentrated in, but not restricted to, the dorsal commissural part of the nucleus of the solitary tract at the level of the apex of calamus scriptorius, and extending into the dorsal subnucleus at the level of the area postrema. Preferential ramifications of carotid sinus and aortic depressor nerve afferents at the levels of the commissural part of the nucleus and the area postrema, respectively, were reflected in the extent to which labeled fibers comingled with neurons exhibiting phenylephrine-induced Fos in dual labeling experiments. Complete sinoaortic denervation reduced by 90% the number of neurons exhibiting drug-induced Fos expression. Selective carotid and aortic sinus denervations effected partial reductions manifest preferentially in the caudal and rostral foci of the distribution, respectively. Reduced activational responses at the level of the area postrema of aortic sinus-denervated rats were accompanied by a reduction in cellular nicotinamide adenine dinucleotide phosphate-diaphorase activity in this region. Animals killed 30 days after complete sinoaortic denervation displayed no evidence of recovery of phenylephrine-induced Fos, while the strength and distribution of the response in rats that received selective carotid sinus denervation were indistinguishable from those seen in controls. These findings (i) support the dependence of phenylephrine-induced Fos expression on the integrity of carotid sinus and aortic depressor nerve afferents, (ii) provide anatomical and functional evidence that the two buffer nerves distribute differentially within the nucleus of the solitary tract, and (iii) implicate central reorganization as a likely basis for functional recovery of baroreflex mechanisms following partial sinoaortic denervation.

Nitric oxide producing neurones in the rat medulla oblongata that project to nucleus tractus solitarii

The production of nitric oxide in neurones of the rat medulla oblongata that project to the nucleus tractus solitarii (NTS) was examined by simultaneous immunohistochemical detection of nitric oxide synthase (NOS) and of cholera toxin B-subunit (CTb), which was injected into the caudal zone of the NTS. Neurones immunoreactive for CTb and neurones immunoreactive for NOS were widely co-distributed and found in almost all the anatomical divisions of the medulla. Dual-labelled cells, containing both CTb and NOS immunoreactivities were more numerous ipsilaterally to the injection sites. They were concentrated principally in the more rostral zone of the NTS, raphé nuclei, dorsal, intermediate and lateral reticular areas, spinal trigeminal and paratrigeminal nuclei and the external cuneate and medial vestibular nuclei. Isolated dual-labelled neurones were also scattered throughout most of the divisions of the reticular formation. These observations indicate that many areas of the medulla that are known to relay somatosensory and viscerosensory inputs contain NOS immunoreactive neurones that project to the NTS, and may, therefore, contribute to the dense NOS-immunoreactive innervation of the NTS. The release of nitric oxide from the axon terminals of these neurones may modulate autonomic responses generated by NTS neurones in relation to peripheral sensory stimuli, and thus ultimately regulate sympathetic and/or parasympathetic outflow.

Pressor response to pulsatile compression of the rostral ventrolateral medulla mediated by nitric oxide and c-fos expression

It has been reported that neurovascular compression of the rostral ventrolateral medulla might be causally related to essential hypertension. Recently, we found that pulsatile compression of the rostral ventrolateral medulla increases sympathetic nerve activity and elevates arterial pressure via activation of glutamate receptors in rats. We also found that increases in sympathetic and cardiovascular activities by microinjection of L-glutamate into the rostral ventrolateral medulla are mediated by c-fos expression-related substance(s) following activation of the nitric oxide-cyclic GMP pathway. Herein, we investigated whether responses to pulsatile compression are mediated by local activation of the nitric oxide-cyclic GMP pathway and/or c-fos expression-related substance(s) in rats. Increases in arterial pressure (15+/-1 mmHg), heart rate (9+/-1 b.p.m.), and sympathetic nerve activity (% change: 8.5+/-1.1%) induced by pulsatile compression were partially but significantly inhibited after local microinjection of a nitric oxide synthase inhibitor, L-N(G)-nitroarginine methyl ester (8+/-2 mmHg, 1+/-1 b.p.m., 4.0+/-1.3%; P<0.05 vs compression without pretreatment) or 7-nitroindazole (7+/-2 mmHg, 2+/-1 b.p.m., 4.0+/-1. 5%; P<0.05), or a soluble guanylate cyclase inhibitor, methylene blue (9+/-1 mmHg, 4+/-1 b.p.m., 4.1+/-1.4%; P<0.05). In addition, increases in arterial pressure, heart rate, and sympathetic nerve activity by pulsatile compression were significantly reduced 6 h after microinjection of antisense oligodeoxynucleotide to c-fos mRNA (2+/-2 mmHg, 2+/-1 b.p.m., 1.0+/-1.0%; P<0.05 vs sense oligodeoxynucleotide). These results suggest that increases in sympathetic and cardiovascular activities induced by pulsatile compression of the rostral ventrolateral medulla are mediated, at least in part, by local activation of the nitric oxide-cyclic GMP pathway and c-fos expression-related substance(s) in rats.

The distribution of neuronal nitric oxide synthase in the nucleus tractus solitarii of the squirrel monkey

The distribution of neuronal nitric oxide synthase (nNOS) containing neurons and fibers in subnuclei of the nucleus tractus solitarii (NTS) in the squirrel monkey, Saimuri sciureus, was investigated by nNOS immunohistochemistry and nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry. Generally, the staining pattern of nNOS and NADPH-diaphorase in the NTS was similar. A high density of neurons and fibers exhibiting both nNOS immunoreactivity and NADPH-diaphorase reactivity was present in the central, medial, intermediate, and dorsolateral subnuclei of the NTS. A moderate density of neurons and fibers that stained for both nNOS and NADPH-diaphorase was noted in the interstitial and ventromedial subnuclei. The gelatinosus and commissural subnuclei contained a low density of neurons and fibers exhibiting nNOS immunoreactivity and NADPH-diaphorase staining. The dorsal motor nucleus of vagus contained a high density of nNOS immunopositive and NADPH-diaphorase containing neurons and fibers at the rostral level, but contained a moderate density of positive fibers and very few positive neurons at the intermediate, subpostremal and commissural NTS levels. Incongruence was noted, however, between nNOS immunostaining and NADPH-diaphorase staining in blood vessels in the brainstem. Capillaries and small vessels exhibited strong staining for NADPH-diaphorase but no nNOS immunoreactivity. In summary, this work substantiates the presence of nNOS in subnuclei of the monkey NTS and is consistent with a role for NO(.) in neurotransmission in primate NTS.

The area postrema of newborn swine is activated by hypercapnia: relevance to sudden infant death syndrome?

This study was performed to investigate a role of the neonatal area postrema (AP) in the chemoreceptor response to hypercapnia which is defective in sudden infant death syndrome (SIDS). AP responses to CO2 inhalation were monitored in 1 to 5 week old piglets by mapping neurons that were induced to express the c-fos gene product, Fos--a marker of functional activation. Interpretive confounds were minimized by controlling for hypoxia, the effects of surgical procedures and ambient environmental stressors on neuronal activity (c-fos expression). The AP demonstrated a powerful and reproducible response in neonatal swine breathing 10% CO2 for 1 h. Intensely immunolabeled nuclei were detected throughout the longitudinal extent of the circumventricular organ, and were especially heavily concentrated at rostral levels proximal to obex. Quantitative analysis verified statistically significant increases in numbers of cells that were induced to express Fos-like immunoreactivity (FLI) in the AP of CO2- stimulated piglets as compared to control groups. No detectable age-related differences were observed in AP response patterns. Conclusions. The AP responds to hypercapnic stress in the newborn piglet. A mature circumventricular organ response in the neonate may be crucial in defending against common environmental stressors, such as nicotine exposure--an emetic agent acting via the AP and a major risk factor in SIDS. Hence, a defect of the AP or its network may underlie a loss of state-dependent controls over cardiopulmonary reflex function in SIDS.